Date: Tue, 9 Feb 2016 09:38:16 -0500
From: "john@kk9a.com" <john@kk9a.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] antenna material
I use 6061-T6 for everything however 6063-T8 may be similar and DX
Engineering sells this with a .120 wall for easier telescoping.
The ARRL antenna book also has element taper scheduled for various wind
speeds. I have been using their 120MPH element designs for years.
John KK9A
## Both 6061-T6 and also 6063-T832 both are 40 ksi yield strength.
Section modulus X yield strength = bending moment..which is all you care
about.
## Dx eng sells the larger diam Al in .120 wall...which is 6061-T8 ( not
T6).
This stuff is slick, since it has a .120 wall.....and the pieces just slide
together.... no
more requirement to machine the ends in a lathe etc...pita. Steel tubing has
been
available in .120 wall for years now.... dunno why it took so long to do the
same
with al tubing.
## EG: 1.75 inch OD x .120 wall will slide right into 2.0 inch OD x .120
wall.
1.5 inch OD x .120 wall, will slide right into 1.75 OD. 1.25 will slide
right into
1.5 etc, etc.
## IF building a full size 40m eles, reduce the tips right down to .0375
inch OD. ( .058 wall material).
## ON4UN ‘forgot’ to tell you his 3 el 40m yagi weighs 350 kg = 770 lbs.
He also forgot to tell you he spliced the inner sections of the ele halves with
a 9 ft long
piece of chromolly steel tubing. No wonder the damned thing is so heavy.
## 2 inch OD is way too small a diam for the center section of a full size
40m yagi... well maybe not.
Splice the inner 2 inch OD sections with an internal sleeve made from 1.75 OD.
Then the
inner 12 ft would be .240 inch thick. IE: use 2 x 12 ft sections of 2.0
OD x .120 wall...and splice
em together with a 12 ft section of 1.75 x .120 wall. Then start tapering
down from there.
## If u design it right, you wont require swedges, machined inserts, etc.
## id use channel aluminum to make each ele to boom bracket. The wide
portion of the channel is called
the ...web. The web is typ available in both .181 thick..and also .281
thick. The short vert sides on the channel
are typ .25 inch thick.
## IMO, Id use al rivets for the ele construction...using .125 diam for the
outer sections...and .187 and also
.250 diam rivets for the heavy wall inner sections. Use SS bolts and SS
nylocks for the inner most splices.
## design it using software..for the mech portion. Then you will know
exactly what wind speed it will be good for,
with or with out ice. Then also factor in sag...and design any truss for
each ele at the same time. Done right,
you can chop a lot of weight out of each ele...and still end up with a robust
ele that will survive.
Jim VE7RF
Ji
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